An inedited laboratory technique combined with 3D numerical modeling was used to investigate attenuation in a partially saturated Berea sandstone sample. The workflow consists in measuring both transient fluid pressure and attenuation in extensional mode at frequencies between 1 and 100 Hz. Measurements for large water saturation such as 97% show a significantly frequency-dependent attenuation at room pressure and temperature. Biot's equations of consolidation are solved with the finite-element method to verify that the observed frequency-dependent attenuation is caused by wave-induced fluid flow on the mesoscopic scale. Both measurements of transient fluid pressure and attenuation could be reproduced by numerical results using a patchy saturated model having same total saturation as the laboratory rock sample. This demonstrated that the measured transient fluid pressure, resulting from fluid flow, was the cause for the fluid-related attenuation observed in the laboratory measurements. We conclude that wave-induced fluid flow on the mesoscopic scale is the dominant mechanism for seismic attenuation in partially saturated Berea sandstone at room pressure and temperature.